1. Field of the Invention
This invention relates to an apparatus for reproducing an image signal recorded on a recording medium.
2. Description of the Related Art
Conventional systems for recording an image signal on recording media include an electronic still video system.
In this electronic still video system, a still image signal recorded on a magnetic disk, called a video floppy disk, is reproduced and displayed on a TV monitor or printed out by a video printer.
In the electronic still video system, the resolution of the still image recorded on the video floppy disk is dependent on the number of pixels of an image sensor (e.g., a charge-coupled device of the like) of a video camera unit used to generate the still image signal.
That is, the horizontal resolution of the still image signal is determined by the number of horizontal pixels of the image sensor whereas the vertical resolution is determined by the number of scanning lines defined by a current TV signal method (e.g., the NISC method).
Accordingly, it has been proposed to improve the resolution by employing two image sensors, each having 500 pixels in the vertical direction and 1200 pixels in the horizontal direction. In this system, the two image sensors are disposed at positions which are shifted from each other in the vertical direction by a distance corresponding to one half pixel relative to an optical image to be formed, and the image signals respectively produced by these image sensors are used to generate a high-resolution still image signal. The generated still image signal is recorded on the recording medium, and the recorded still image signal is reproduced.
FIG. 1 shows the positional relationship between the pixels of the first and second image sensors employed in the above-described system.
Sampling is conducted on the pixels indicated by o in FIG. 1 in the image signals output from the respective image sensors, and the sample image signal is recorded on the video floppy disk by using a two-channel magnetic head to form four tracks.
FIG. 2 shows the recording pattern of the four tracks formed on the video floppy disk. In this pattern, the image signal output from the first image sensor is recorded on tracks A and C, and the image signal output from the second image sensor is recorded on tracks B and D.
For reproduction, the image signal recorded on the four tracks on the video floppy disk in the abovedescribed manner is reproduced, the reproduced image signal is reconstructed in a memory, and interpolation is then conducted on the reconstructed image signal to generate a still image signal, having a horizontal resolution of 1200 pixels and a vertical resolution of 1000 pixels. Thereafter, the generated still image signal will be supplied to a TV monitor or a video printer to display or print out a high-resolution still image.
A transmission system, in which part of the image signals output from the image sensors is extracted, in which the extracted signal is recorded on the video floppy disk and in which the signals reproduced from the video floppy disk are reconstructed in the memory, as in the case of the above-described s).stem, constitutes one type of analog transmission of a sampled value. In such a transmission system, the total amplitude and phase characteristics must be strictly controlled, and time base variations generated in the recording and reproduction system must be accurately corrected in order to conduct re-sampling of the reproduced image signal when the reproduced image signal is reconstructed.
It has accordingly been proposed to correct the time base variations in the manner described below: during recording, a time base correcting pilot signal is frequency-multiplexed with the image signal between a color-difference frequency-modulated signal band and a luminance frequency-modulated signal band, as shown in FIG. 3. During reproduction, a sampling clock signal having the same jitter component as that of the reproduced pilot signal is formed, and the reproduced image signal is sampled and then stored temporarily in the memory or the like by using that sampling clock signal. Thereafter, the stored image signal is read out by using a clock signal having no time base variations.
However, in the above-described electronic still video system, since the image signal wi&h which the pilot signal is frequency-multiplexed is recorded on the video floppy disk while it is rotating at a predetermined rotational speed, the recording wavelength for the outer circumference tracks on the video floppy disk varies from that for the inner circumference tracks, so that the phase difference between the reproduced image signal and the reproduced pilot signal, generated due to the non-linear distortion generated in an electromagnetic conversion system constructed by the video floppy disk and the magnetic head, varies depending on the track position.
Thus, even if the time base variations are corrected by means of the reproduced pilot signal, since the phase difference varies between the reproduced image signal and the reproduced pilot signal, which is generated on the outer circumference tracks on the video floppy disk, varies from the reproduced signals generated on the inner circumference tracks, sampling cannot be conducted in the reproduction in the same timing in which sampling is conducted during recording. The image signal cannot, therefore, be restored with a high degree of accuracy.
Furthermore, in the above-described electronic still video system, when the frequency-modulated image signal reproduced from the video floppy disk is demodulated and stored in the memory, the horizontal address of the image signal storing memory is generally reset in synchronism with the horizontal and vertical synchronizing signals affixed to the demodulated image signal. However, if waveform distortion occurs in the synchronizing signal portion of the demodulated image signal or if the SN deteriorates, as shown in FIG. 4(a), shaping of the waveform of that portion produces a wave form, the time base of the edge portion of which deteriorates, as shown in FIG. 4(b). In consequence, if that synchronizing signal is used to reset the horizontal address of the image signal storing memory, the image signal will be stored at an address in the memory different from that at which it would be stored during the normal operation. In that case, even if a precise reading-out clock signal is used to read out that image signal stored in the memory in that manner, the restored image is deformed due to that shift of the address, and an original image cannot thus be restored with a high degree of accuracy.